1. Field of the Invention
This invention relates to the surface mounting of leadless components on a substrate supporting a conductor pattern, eg. so-called printed circuits.
2. Description of Related Art
It is known to apply solder cream (a mixture of solder particles, flux, solvent and rheological components) to the component connection pads of a printed circuit, place the components, partially cure the cream (warm it in an oven so that it dries) which action adheres the components lightly to the connection pads, and then subsequently to subject the assembly to a solder reflow process (by any of various means, eg. hot gas, vapour phase, infra-red radiation or conductive heating) which melts the solder of the solder cream and fixes the component finally in position.
The main problem with the above technique is that there is left a very small gap, perhaps one to three thousandths of an inch, between the component and the circuit substrate. Flux is drawn into this gap by capilliary action and is then very difficult if not impossible to clean out, ie. so it remains to seep out subsequent to manufacture and perhaps corrode the printed circuit pattern and the joints on components and assemblies. In addition, the thermal contact between the component and the substrate is poor so that stress may occur due to different heating rates of component and substrate during use of the manufactured item, and in any case, the partially cured solder cream is not a strong adhesive and may not be sufficient to maintain the components in place during handling of the circuit prior to the solder reflow process. Finally, especially for circuits which may be subject to very high shock loads and acceleration during use, the fixing of the component to the substrate by virtue only of the component pads being soldered to the conductor pattern may not be sufficient.